Semiconductor devices such as logic and memory devices are typically fabricated by a sequence of processing steps applied to a specimen. The various features and multiple structural levels of the semiconductor devices are formed by these processing steps. For example, lithography among others is one semiconductor fabrication process that involves generating a pattern on a semiconductor wafer. Additional examples of semiconductor fabrication processes include, but are not limited to, chemical-mechanical polishing, etch, deposition, and ion implantation. Multiple semiconductor devices may be fabricated on a single semiconductor wafer and then separated into individual semiconductor devices.
Optical metrology processes are used at various steps during a semiconductor manufacturing process to detect defects on wafers to promote higher yield. Optical metrology techniques offer the potential for high throughput without the risk of sample destruction. A number of optical metrology based techniques including scatterometry and reflectometry implementations and associated analysis algorithms are commonly used to characterize critical dimensions, film thicknesses, composition and other parameters of nanoscale structures.
As devices (e.g., logic and memory devices) move toward smaller nanometer-scale dimensions, characterization becomes more difficult. Devices incorporating complex three-dimensional geometry and materials with diverse physical properties contribute to characterization difficulty.
In response to these challenges, more complex optical tools have been developed. Multiple, different measurement technologies are available, and measurements are performed over a large ranges of several machine parameters (e.g., wavelength, azimuth and angle of incidence, etc.), and often simultaneously. As a result, the measurement time, computation time, and the overall time to generate reliable results, including the synthesis of measurement libraries, increases significantly.
Library based measurements employ one or more pre-computed functional models that relate values of parameters of interest to measurement data (e.g., spectra). The pre-computed functional model approximates the solution of Maxwell's equations for a given set of parameter values. The library is synthesized from training data prepared by theoretical analysis and calculation of subsystem configuration, signal sensitivities, geometric models, etc. The process of building a measurement library is expensive in time and computational effort.
In many instances, the measurement library is validated based on measured reference data. Reference measurement data is typically collected from trusted measurement instruments, such as a transmission electron microscope, a scanning electron microscope, an atomic force microscope, scanning tunneling microscope, an x-ray based metrology system, etc., and the results are compared with estimates provided by the measurement library.
Typically, the estimates provided by the measurement library do not match the reference data within specified margins for early measurement library iterations. The synthesis of the measurement library often does not involve a significant amount of real fabrication process knowledge. This leads to modeling inaccuracies in early measurement library iterations that must be corrected. Thus, additional effort must be expended to study the root cause, modify the geometric model, and regenerate the library. This results in a substantial loss of time and computational effort. In addition, measured reference data is typically available only at the very late stages of a development cycle. Thus, the delays associated with library regeneration have a significant impact of production schedules.
As the available range of optical metrology measurement subsystems and associated recipes has increased, so has the complexity of the measurement selection process. Improved methods and tools to optimize the use of precomputed library functions to perform accurate library based measurements are desired.